MIMO Systems
It is well known that MIMO systems can significantly increase the data carrying capacity of wireless systems. Multiple antennas for transmission and reception are used for improving both the user- and cell throughput and are key factors behind the high performance offered by 3GPP (3rd generation partnership program) LTE (long-term evolution) standard. Starting from Rel-10 up to 8 layers is supported, see e.g. Sec. 4.2.1 in 3GPP TS 36.201, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); LTE physical layer; General description, Release 10, V10.0.0. However, the UE (User Equipment) performance requirements are still based on the use of 2 receive antenna ports (AP); there are no requirements for a UE that can be equipped with more than two antenna ports for achieving additional diversity gain and/or multiplexing gain.
With 4 Rx (Mx4) MIMO system (e.g. 4×4 MIMO or 8×4 MIMO i.e. M=4 or 8), up to four layer spatial multiplexing is supported. With 4 Rx AP (antenna ports) an 8×4 MIMO system with four layer spatial multiplexing is capable of utilizing both beam forming and diversity gain in maximum level. These layers can be combined through dynamic beamforming and MIMO receiver processing to increase reliability of the received signal at the UE and the range of the UE in the cell. From a performance point of view the use of 4 Rx AP allows higher UE data rates in a wide range of scenarios and improved receiver sensitivity in general. Depending on the target signal quality at the UE (e.g. SNR or SINR region), the transmission scheme (e.g. 4×4 MIMO) used in the network node (e.g. eNodeB) and the channel conditions, the peak user throughput can be doubled compared to dual-layer multiplexing by virtue of additional receiver diversity gain and/or multiplexing gain at the UE. Additionally, due to the improved UE receiver sensitivity, cell coverage may under ideal circumstances be doubled, resulting in fewer blind spots entirely lacking signal reception at the UE. The improved UE receiver sensitivity herein means for example that the minimum mean power received at the UE capable of 4 Rx can be lower than that received at the UE capable of less than 4 Rx e.g. 1 Rx or 2 RX.
Note that terminology such as NodeB or eNode B and UE should be considering non-limiting and does in particular not imply a certain hierarchical relation between the two, in general “NodeB” could be considered as device 1 and “UE” device 2, and these two devices communicate with each other over some radio channel. Herein, we also focus on wireless transmissions in the downlink, but some embodiments are equally applicable in the uplink.
The term “network node” is used in some parts of this disclosure as a generic term for bases stations, such as NodeB or eNodeB. Furthermore, the term “wireless device” is used in some parts of this disclosure as a generic term for devices such as UEs.
Interference Mitigation
Interference mitigation is a technique used to at least partly mitigate inter-cell interference. In the UE the inter-cell interference mitigation receiver at least partly mitigates interference caused by the one or more radio signals transmitted by one or more interfering cells aka aggressor cells, neighbor cells etc.
The terms interference mitigation (IM) receiver, interference cancellation (IC) receiver, interference suppression receiver, interference rejection receiver, interference aware receiver, interference avoidance receiver, or any combination thereof are interchangeably used but they all belong to a category of an advanced receiver or an enhanced receiver. Interference cancellation or suppression by such advanced receiver structures can lead to the elimination of the interference, in which case the interference is completely cancelled, whereas in other cases the impact of interference on the useful signal is reduced. Hereinafter for the sake of consistency the term IM is used.
Examples of useful signals, which are intended to be received at the UE, are data channel (e.g. PDSCH), control channels (e.g. PDCCH, EPDCCH etc), common channel (e.g. PBCH), physical signals such as reference signals or pilot signals (e.g. CRS, PRS, discovery signals, PSS, SSS etc).
Examples of signals or channels whose interference from one or more interfering cells at the UE can be mitigated by the UE in LTE are PDSCH, PDCCH, PCFICH, PCFICH, EPDCCH, PBCH, CRS, PRS, etc. Examples of signals or channels whose interference from one or more interfering cells at the UE can be mitigated by the UE in HSPA are HS-PDSCH, HS-SCCH, P-CPICH, S-CPICH, DPCCH, F-DPCCH, etc.
An example of baseline receiver, which does not mitigate interference from interfering cells, is MMSE-MRC.
Examples of IM receivers which can be used for mitigating interference caused by data or control channels (e.g. PDSCH, PDCCH/PCFICH) transmissions in one or more interference cells are MMSE-IRC, E-MMSE-IRC, R-ML, CWIC, iterative ML etc.
Examples of IM receivers which can be used for mitigating interference caused by physical signals (e.g. discovery signals, CRS, PSS, PRS, SSS etc) transmissions in one or more interference cells are reference signal IM such as CRS-IM (aka CRS-IC) etc.
In practice the UE may also apply combination of IM receivers to mitigate interference caused by one or more interfering cells. For example a UE may mitigate interference caused by PDSCH as well as CRS transmissions in an interfering cell (e.g. cell2) when receiving a control channel (e.g. PDCCH) from the serving cell (e.g. cell1). As an example the UE may use combination of IM receivers comprising of MMSE-IRC, CWIC and CRS-IM to mitigate interference caused by control/data, PDSCH and CRS transmissions from cell2.
DRX Cycle Operation
In LTE DRX (Discontinuous reception) cycle is used to enable UE to save its battery. The DRX cycle is used in RRC idle state but it can also be used in RRC connected state. When operating in DRX or using DRX cycle, the UE is interchangeably called being in ‘DRX state’ or ‘DRX mode’ or ‘DRX operation’ or ‘DRX operational state or mode’. For consistency hereinafter, the term DRX state is used.
Examples of lengths of DRX cycles used in RRC idle state 320 ms, 640 ms, 1.28 s and 2.56 s. Examples of lengths of DLX cycles used in RRC connected state may range from 2 ms to 2.56 s. The DRX cycle is configured by the network node and is characterized by the following parameters:                On duration: During the on duration of the DRX cycle, a timer called ‘onDurationTimer’, which is configured by the network node, is running. This timer specifies the number of consecutive control channel subframes (e.g. PDCCH, ePDCCH subframe(s)) at the beginning of a DRX Cycle. It is also interchangeably called DRX ON period. More specifically it is the duration in downlink subframes that the UE after waking up from DRX to receive control channel (e.g. PDCCH, ePDCCH). If the UE successfully decodes the control channel (e.g. PDCCH, ePDCCH) during the ON duration then the UE starts a drx-inactivity timer (see below) and stays awake until its expiry. When the onDurationTimer is running the UE is considered to be in DRX state of the DRX cycle.        drx-inactivity timer: It specifies the number of consecutive control channel (e.g. PDCCH, ePDCCH) subframe(s) after the subframe in which a control channel (e.g. PDCCH) indicates an initial UL or DL user data transmission for this MAC entity. It is also configured by the network node. When the drx-inactivity timer is running the UE is considered to be in non-DRX state i.e. no DRX is used.        Active time: This time is the duration during which the UE monitors the control channel (e.g. PDCCH, ePDCCH). In other words this is the total duration during which the UE is awake. This includes the “on-duration” of the DRX cycle, the time during which the UE is performing continuous reception while the inactivity timer has not expired and the time the UE is performing continuous reception while waiting for a DL retransmission after one HARQ RTT. The minimum active time is equal to the length of an on duration, and the maximum active time is undefined (infinite).        
The DRX ON and DRX OFF durations of the DRX cycle are shown in FIG. 1. The DRX operation with more detailed parameters in LTE is illustrated in FIG. 2.